The market for microelectromechanical systems (MEMS) has grown rapidly since their introduction in the 1990's with the development of airbag accelerometers and micro-mirror array chips. Often, these devices are uniquely capable of complex mechanical tasks. Within the last decade complex, integrated MEMS have been developed that comprise motors, movable mirror arrays, mechanical switches, integrated circuits (ICs), and fluidic flow channels. Small sensors and communication chips are being developed using similar technologies.
Formation of a MEMS device can include surface and bulk micromachining processes. Surface micromachining comprises building up, layer by layer, the MEMS device structure on the surface of a substrate. In particular, polysilicon surface micromachining uses the planar fabrication techniques common to microelectronic circuit fabrication to manufacture the MEMS device. The multiple layers are built up on the substrate (e.g., a single crystal silicon wafer) using thin film depositions of low-stress polycrystalline silicon (poly-Si) as a structural material and silicon dioxide (SiO2) as a sacrificial material. Silicon dioxide is a preferred sacrificial material due to its wide use in IC processing, thermal compatibility during the high temperature deposition and anneal of poly-Si, and because it can be selectively etched with respect to silicon and some metals. In addition, SiO2 is a dielectric material that can be used for electrical insulation. SiO2 layers can be thermally grown or deposited by chemical vapor deposition. Photolithographic patterning and dry etching are generally used to define surface features in the deposited structural layers in a plane parallel to the substrate. Vias etched through the sacrificial layers provide anchor points between the structural layers and the substrate. Wet etching with a selective etchant can then be used to remove the sacrificial oxide layers and release the MEMS device. Alkaline SiO2 etchants also react with silicon. Therefore, acid etchants comprising hydrogen fluoride (HF) are typically used to selectively etch the SiO2 sacrificial layers. Typically, HF is highly selective between the sacrificial oxide and the poly-Si structural layers, and has a high etch rate. However, oxide etch rate and selectivity can depend on a number of factors, including the presence of chemical impurities (e.g., dopant concentration, etchant contaminants, etc.), film density, intrinsic stress, microstructure, etc.
Many hundreds of fully assembled MEMS devices can thereby be batch-fabricated on a single silicon substrate. Quite complex structures can be built up using the polysilicon surface micromachining process described above, enabling the construction of highly functional MEMS devices. The total stack height of the MEMS device structure can be about 10 microns or greater. A typical polysilicon surface micromachining process is the five-level polysilicon process described in U.S. Pat. No. 6,082,208 to Rodgers et al., and J. J. Sniegowski and M. P. de Boer, “IC-Compatible Polysilicon Surface Micromachining,” Ann. Rev. Mater. Sci. 30, 299 (2000), which are incorporated herein by reference.
A production challenge that faces the development of a new generation of devices is the addition of noble metal films to MEMS. The use of noble metal films permits the fabrication of MEMS with greater complexity and functionality. For example, noble metal films may be used to add highly reflective optical surfaces, electrically conductive lines, bond pads, switch posts, electrodes, or long-lasting structures. However, the use of concentrated hydrofluoric acid during the release etch on such silicon/metal devices almost always causes problems. It is not uncommon for the poly-Si structural layers to dissolve or experience surface roughening in the presence of noble metal films. Furthermore, metal films will often delaminate during the release etch step due to undercutting of the underlying structural layer. Thus, etchant solution chemistries that prevent the degradation of structural layers during the release etch step are highly sought after.